Not applicable.
Not applicable.
This invention relates to alkanolamine sweetening facilities for processing gases containing ammonia and/or a cyanide, and more particularly to methods for neutralizing corrosive constituents that accumulate in the reflux stream of such facilities.
Alkanolamine sweetening facilities are commonly used to remove H2S and CO2 from a variety of gases, including natural gases, enhanced oil recovery gases, refinery hydrodesulfurizer recycle gases, FCCU and Coker gas plant tail gases, LPG streams and Claus sulfur recovery tail gases. Such facilities include an absorber, in which H2S and CO2 are dissolved in an aqueous solution of an alkanolamine to form solvated alkanolamine salts (erg., protonated alkanolamine cation with HSxe2x88x92 and HCO3xe2x88x92 anions). The alkanolamine salts of H2S and CO2 are not heat stable and are decomposed by steam stripping in a stripping column, with the concomitant removal of the released H2S and CO2 and deprotonation of the alkanolamine, freeing it for repeated reaction with acid gas in the absorber.
Unfortunately, alkanolamine salts are also formed with anions of other strong and weak acids that accumulate in the circulating solution. These may derive from gases, such as SO2, COS, or HCN, which are present in the hydrocarbon gases through reactions in the alkanolamine solution. These additional alkanolamine salts cannot be removed by steam stripping, unlike H2S and CO2 salts, and thus they are called heat stable salts. Heat stable salts remain in the system where they accumulate in the alkanolamine solution, gradually depleting the effectiveness of alkanolamine treatment. In addition, high concentrations of heat stable anions in the alkanolamine solution corrode the carbon steel components of the system. The corrosion products are also known to contribute to foaming problems in the system which further decreases treating effectiveness and causes amine losses. Various attempts have been made at solving these problems. See, e.g., Nielsen et al., AControlling Corrosion in Amine Treating Plants,@ Proc. Laurance Reid Gas Cond. Conf. 45th, pp. 182-212 (1995).
It has been proposed to remove heat stable salts from the lean alkanolamine solution exiting the bottom of the stripping column by ion exchange filtration. See, e.g., U.S. Pat. Nos. 2,797,188, 4,122,149, 4,170,628, 4,477,419, 4,758,311, 4,795,565, 4,970,344, 4,999,113, 5,006,258, 5,162,084, 5,277,822 and 5,788,864. Generally, heat stable salt anions are removed by exchange with hydroxide from an anion exchange resin and cations, such as sodium and potassium, are removed by exchange with hydrogen ion from a cation exchange resin. In the anion case, the protonated amine from the heat stable salt is deprotonated by reaction with hydroxide from the resin, resulting in water and free amine capable of reacting with acid gases.
Despite the foregoing developments, there is still room for improvement in the art. In particular, it is desired to reduce the amount of alkanolamine lost in the alkanolamine regeneration process. It is further desired to extend the time that the ion exchange resin can be onstream prior to regeneration. It is still further desired to provide a method for neutralizing corrosive constituents that can accumulate in the stripping column reflux stream of an alkanolamine facility processing synthesis gas feedstocks containing ammonia and or cyanides.
All references cited herein are incorporated herein by reference in their entireties.
Accordingly, a process is provided for removing CO2 and/or H2S from a gas mixture containing as impurities CO2 and/or H2S and at least one other impurity selected from the group consisting of a cyanide and/or ammonia, the process comprising:
conveying the gas mixture to an absorber vessel in which the gas mixture is contacted with an aqueous absorbent solution containing an acid gas removal solvent, such as an alkanolamine, to remove CO2 and/or H2S from the gas mixture and form a rich aqueous absorbent solution enriched in CO2 and/or H2S removed from the gas mixture;
conveying the rich aqueous absorbent solution from the absorber vessel to a stripping vessel in which CO2 and/or H2S are thermally stripped from the rich aqueous absorbent solution to form a lean aqueous absorbent solution depleted of CO2 and/or H2S, and an overhead vapor stream rich in CO2 and/or H2S and the at least one other impurity;
recycling the lean aqueous absorbent solution from the stripping vessel to the absorber vessel to absorb additional amounts of the gas mixture;
cooling the overhead vapor stream in a heat exchanger and separating the resulting two phase stream in a separator vessel to provide an acid gas rich vapor stream and an alkanolamine rich aqueous solution; and
contacting at least a portion of the alkanolamine rich aqueous solution with an anion exchange resin to remove from the alkanolamine rich aqueous solution at least a part of at least one other impurity prior to recycling the alkanolamine rich aqueous solution for further use in the process.
Also provided is an apparatus adapted to perform the process of the invention. A preferred embodiment of the apparatus of the invention comprises:
an absorber vessel comprising a sour gas mixture inlet and a rich aqueous absorbent solution outlet below a purified gas outlet and an acid gas removal solvent rich aqueous solution inlet;
a stripping vessel comprising in order from top to bottom, an overhead vapor stream outlet, a recycled reflux inlet, a rich aqueous absorbent solution inlet and a lean aqueous absorbent solution outlet, wherein said rich aqueous absorbent solution inlet is in fluid communication with said rich aqueous absorbent solution outlet and said lean aqueous absorbent solution outlet is in fluid communication with said acid gas removal solvent rich aqueous solution inlet;
a heat exchanger comprising an overhead vapor stream inlet and a two phase outlet on one side and a coolant stream inlet and a coolant stream outlet on the other side, wherein said overhead vapor stream inlet is in fluid communication with said overhead vapor stream outlet;
a separator vessel comprising in order from top to bottom, an acid gas rich stream outlet, a two phase stream inlet, and an acid gas removal solvent rich aqueous solution outlet, wherein said two phase stream inlet is in fluid communication with said two phase stream outlet and said acid gas removal solvent rich aqueous solution outlet is in fluid communication with said recycled reflux inlet; and
a resin bed comprising said anion exchange resin, a resin bed inlet, and a resin bed outlet, wherein said resin bed inlet is in fluid communication with said acid gas removal solvent rich aqueous solution outlet, and said resin bed outlet is in fluid communication with said acid gas removal solvent rich aqueous solution inlet.
An alternative embodiment of the apparatus comprises:
an absorber vessel comprising a sour gas mixture inlet and a rich aqueous absorbent solution outlet below a purified gas outlet and an acid gas removal solvent rich aqueous solution inlet;
a stripping vessel comprising in order from top to bottom, a rich reflux gas outlet, a recycled reflux inlet, a rich aqueous absorbent solution inlet and a lean aqueous absorbent solution outlet, wherein said rich aqueous absorbent solution inlet is in fluid communication with said rich aqueous absorbent solution outlet and said lean aqueous absorbent solution outlet is in fluid communication with said acid gas removal solvent rich aqueous solution inlet;
a heat exchanger comprising an overhead vapor stream inlet and a two phase outlet on one side and a coolant stream inlet and a coolant stream outlet on the other side, wherein said overhead vapor stream inlet is in fluid communication with said overhead vapor stream outlet;
a separator vessel comprising in order from top to bottom, an acid gas rich stream outlet, a two phase stream inlet, and an acid gas removal solvent rich aqueous solution outlet, wherein said two phase stream inlet is in fluid communication with said two phase stream outlet; and
a resin bed comprising said anion exchange resin, a resin bed inlet, and a resin bed outlet, wherein said resin bed inlet is in fluid communication with said acid gas removal solvent rich aqueous solution outlet, and said resin bed outlet is in fluid communication with said recycled reflux inlet.